1. Field of the Invention
The invention relates to display devices, and in particular to electrowetting display devices and fabrication methods thereof.
2. Description of the Related Art
Electrowetting display devices are rendered images in accordance with electrowetting or electrocapillary. Briefly, the free surface energy of some fluids is changed due to electric field effects such that distribution area of the fluids can thus change along with the electric field effects.
PCT publication No. WO 2005/051091, the entirety of which is hereby incorporated by reference, discloses a structure of an electrowetting display device. Referring to FIG. 1, a conventional electrowetting display device comprises a second substrate 3 and a first substrate 4 opposing to each other. A patterned pixel electrode 7 is disposed on the second substrate 3. A dielectric layer 8 such as a material layer with a hydrophobic surface is disposed on the second substrate 3 and the pixel electrode 7. Patterned hydrophilic bank structures 13 (such as pixel walls) are disposed on the dielectric layer 8 defining each pixel region. An opaque non-polar fluid 5 including black dyes is disposed in the pixel region between bank structures 13. A transparent polar fluid 6 is disposed between the gap between the first substrate 4 and the second substrate 3. When the operatic voltage is off, the opaque non-polar fluid 5 is uniformly distributed within each pixel region, thus the display status is a dark state.
When the operatic voltage is on, i.e., an electric field is generated between the first substrate 4 and the second substrate 3 by a voltage source 9, the opaque non-polar fluid 5 is cohered due to an electrowetting effect, thereby exposing most of the pixel region. Thus, the display status is a bright state.
The bank structure of the conventional electrowetting display device is a hydrophilic structure which is directly formed on the dielectric layer 8 with hydrophobic low-surface-energy. It is beneficial that the low-surface-energy material can be entirely applied to and formed on the substrate. However, it is difficult to fabricate the hydrophilic bank structure 13 directly on the dielectric layer 8. The hydrophilic bank structure 13, moreover, is prone to peel off from the low-surface-energy dielectric layer 8 causing display failure.
U.S. Pub. No. 2007/0188676, the entirety of which is hereby incorporated by reference, discloses an electrowetting display device. Coherence of the opaque non-polar fluid is controlled by an operatic electric field due to the electrowetting effect, thereby displaying a bright-state and a dark-state, respectively. FIG. 2 is a cross section of another conventional electrowetting display device. Referring to FIG. 2, the conventional electrowetting display device includes a back light unit 20 and an electrowetting display 50 serving as a light switch. The electrowetting display 50 includes a second substrate 22, a pixel electrode 24 disposed on the second substrate 22, and a dielectric layer (with a hydrophobic surface) 26 disposed on the pixel electrode 24. Patterned hydrophilic bank structures 28 are disposed on the dielectric layer 26 defining each pixel region. An opaque non-polar fluid 45 with black dyes and a transparent polar fluid 40 are disposed within each pixel region. The first substrate 30 with patterned common electrodes 32 disposed thereon is arranged on the bank structure 28 and transparent polar fluid 40 opposing to the second substrate 22.
Furthermore, PCT publication No. WO 2006/017129, the entirety of which is hereby incorporated by reference, discloses a transflective electrowetting display structure in which a second substrate and a first substrate attached with color filters are assembled. A polar fluid and a black non-polar fluid are interposed between the second and first substrate. The transflective color electrowetting display includes a plurality of pixels. Each pixel is divided into a transmission region and a reflective region on the second substrate.
FIG. 3 is a cross section of a conventional transflective electrowetting display. Referring to FIG. 3, the conventional transflective color electrowetting display includes a second substrate 112 disposed on a back light unit 111. A patterned reflector 113 corresponding to the reflective region of each pixel is disposed on the second substrate 112. A second transparent electrode 114 is disposed on the entire region of the second substrate 112 covering the reflector 113. A dielectric layer (with hydrophobic surface characteristics) 115 is disposed on the second transparent electrode 114. A patterned bank structure 116 is disposed on the dielectric layer 115 defining a plurality of pixel regions.
A first substrate 118 is opposing to the second substrate 112. A first transparent electrode 117 is disposed on the first substrate 118. A transparent polar fluid layer 121 and opaque non-polar fluid layers 120a-120c are interposed in the gap between the second substrate 112 and the first substrate 118. A power supply applies a bias between the first transparent electrode 114 and the second transparent electrode 117. An electrowetting force due to the bias causes convergence of the opaque non-polar fluid layer, thereby controlling reflective and transmissive regions of each pixel operation. When the applied voltage exceeds the threshold voltage, the opaque non-polar fluid layer begins to converge gradually exposing both the reflective and transmissive regions.
The second substrate structure of the conventional electrowetting display primarily includes a material layer with low-surface energy and a patterned hydrophilic structure fabricated on the low-surface energy layer. However, since the surface of the low-surface energy layer has anti-adhesion properties, it is difficult to proceed with subsequent large area application and processes. Moreover, even if a hydrophilic structure is firmly formed on the low-surface energy layer, the hydrophilic structure will easily be peeled off from the low-surface energy layer due to its surface characteristics. Therefore, there is a need for a fabricating method, which firmly fabricates a hydrophilic structure on a low-surface energy dielectric layer, overcoming problems associated with the surface characteristics of the hydrophilic structure, so as to improve structural stability of an electrowetting display device.